The present invention relates to an information reproduction device such as optical disk and the like. An increased advantage is obtained from an information reproduction device which reads out multi-valued digital signals having two or more values.
In a conventional information reproduction device, a laser beam emitted from an optical head is focused to form an optical spot on a recording medium. The position of the optical spot on the recording medium is controlled by moving the optical head by the scanner while rotating the recording medium. A readout signal obtained by detecting the light reflected from the recording medium is demodulated by a reproduction method having at least a demodulation circuit thereby to obtain reproduced data. The reproduction operation in general is controlled by a central controller.
A readout signal obtained by the conventional information reproduction device and a demodulation method will be described with reference to FIG. 4. Ideally, a readout signal 11 assumes levels of three values, i.e., 0, 1 and 2. Recorded marks for obtaining such a readout signal are arranged at such a density that a maximum of, for example, two recorded marks are included in an optical spot. Then, the number of recorded marks within the optical spot changes in three values, i.e., 0, 1, 2 accompanying the scanning of the optical spot. It is, therefore, made possible to obtain the readout signal 11 having levels of three values, i.e., 1, 2 and 3 as shown in FIG. 4a. In order to demodulate the recorded data from the readout signal 11, the two reference levels 121 and 122 are set between 2 and 1, and between 1 and 0. The readout signal is compared with the two reference levels to thereby obtain demodulated data of 0, 1 and 2. As required, the data may be further decoded to obtain binary data of 0 and 1.
The above-mentioned prior art has been disclosed in, for example, Japanese Patent Laid-Open No. 236823/1990.
The above-mentioned prior art, however, involves a problem in that demodulation error takes place when the level of the readout signal undergoes a change even when the change is in a low-frequency range being affected by dispersion in the size of the recorded marks as shown in FIG. 4b and by the substrate retardation.
In order to solve the above-mentioned problem, the object of the present invention is to provide a high-density information reproduction method and device that are capable of virtually maintaining the quality of readout signals and suppressing the occurrence of error even when the level of the readout signal has changed due to dispersion in the size of the recorded marks and substrate retardation.
The following means is employed in order to accomplish the object of the present invention.
Referring to FIG. 5, the readout signal 14 is compared with at least one of reference levels 121 and 122 in order to demodulate digital data having two or more values. The reference levels 121 and 122 are set based upon standard levels. Referring to FIG. 5, for instance, the reference level 121 is set to lie between standard levels 131 and 132. The reference level 122 is set to lie between standard levels 132 and 133. In FIG. 5, for instance, a level equidistant from the two standard levels is used as the reference level.
In the present invention, the standard level corresponding to the demodulated digital data is caused to change following the level of the readout signal. In FIG. 5, for instance, the digital signals having three values of 0, 1 and 2 correspond to the standard levels 133, 132 and 131, respectively. When a digital signal demodulated at a given timing is 0, the standard level 133 is changed following a change of the readout signal. As a result, the reference level 122 undergoes a change. When the digital signal demodulated at a given timing is 1, the standard level 132 is changed following the change of the readout signal. As a result, the reference levels 121 and 122 undergo a change. When the digital signal demodulated at a given timing is 2, the reference level 131 is changed following the change of the readout signal. As a result, the reference level 121 undergoes a change.
As described above, any one of the standard level 131, 132 or 133 is caused to change following the level of the readout signal. The reference levels 121 and 122 are changed based on the standard level that has changed following the above-mentioned change.
Even when the level of the readout signal changes, therefore, the reference level to be compared is caused to change following the level of the readout signal at all times. Therefore, the S/N ratio virtually increases making it possible to reproduce information suppressing the error rate down to a practically very low level.
The readout signals may be analog signals or multi-value digital signals having not less than three values. The present invention makes it possible to demodulate even those signals which are usually difficult to demodulate.
The demodulation into the digital data may be executed after every predetermined time interval (e.g., clock). This makes it possible to reliably detect and follow the level of the readout signal even when the frequency response of the signal is delayed behind the channel clock frequency.
Moreover, the time interval may be changed depending upon the comparison of the readout signal with the standard level. It is desired that the timing (phase of detection clock) of comparison of the readout signal with the standard level is changed based upon the difference between the standard level and the level of the readout signal to control it by feedback. This makes it possible to control the time in response to a deviation in the signal phase during the reproduction and, hence, to reproduce information maintaining wider margin.
It is further allowable to employ a drawing step which brings the standard levels of two or more values close to the level of the corresponding readout signal by using, as a data readout signal, a signal that corresponds to the known corresponding data. This makes it possible to reliably set the standard level in an initial state. In this case, it is desired to use, as a readout signal, a signal having a predetermined time duration that enables a known signal to be obtained or to use, as a recording medium, the one having a particular (learning) region that enables a known signal to be obtained.
The following means is employed to constitute the present invention as a device.
That is, an information reproduction device comprising a readout signal detection circuit for detecting a readout signal, one or more reference level generators, and a level comparator for comparing the reference levels generated by the reference level generators with the readout signal, the information reproduction device further comprising two or more standard level holders for holding standard levels, a selector for selecting any one of a plurality of standard levels based upon the result of comparison by a level detector, end level compensators for compensating the standard levels based upon the comparison of the standard level selected by the selector with the level of the readout signal, wherein the reference level generators generate reference levels based upon a plurality of standard levels.
Accordingly, even when the level of the readout signal changes, the reference levels to be compared change following the level of the readout signal at all times, making it possible to virtually improve the S/N ratio and to carry out the reproduction suppressing the error rate down to a practically very low level.
The invention is further concerned with an information reproduction device comprising a timing controller for generating clocks maintaining nearly a constant time interval, a sampling/holding circuit for sampling and holding the readout signal maintaining nearly a constant time interval based upon a clock generated by the timing controller, one or more reference level generators, and a level comparator for comparing the reference levels generated by the reference level generators with the readout signal, the information reproduction device further comprising two or more standard level holders for holding standard levels, a selector for selecting any one of a plurality of standard levels based upon the result of comparison by a level detector, and level compensators for compensating the standard level based upon the comparison of the standard level selected by the selector with the level of the readout signal, wherein the reference level generators generate reference levels based upon a plurality of standard levels.
It is thus made possible to reliably detect and follow the level of the readout signal even when the frequency response of the signals having multi-valued levels is delayed behind the channel clock frequency of the signals.
It is further desired to provide the standard level holders in a number larger than that of the reference level generators. It is thus made possible to easily cope with the readout signals having digital data of three or more values.
It is desired that the timing controller has a function for controlling the phase of the clocks based upon the level compensation data from the level compensators. This makes it possible to control the time in response to a deviation in the phase of the signals during the reproduction and, hence, to carry out the reproduction maintaining a wider margin.
In the present invention, the reference levels that serve as standards for comparison at the time of demodulating the readout signal are varied with the change of readout signal. A plurality of standard levels corresponding to the level of the readout signal are corrected according to the data demodulated from the readout signal so as to be in agreement with the level of the readout signal detected at a given moment. Even when the signal level changes, therefore, the data can be demodulated without any increase in the error rate provided the change of the signal level is within a range in which the standard level can follow.